Free machining austenitic stainless steel alloy

ABSTRACT

Austenitic stainless steel alloy containing about 10 percent to 26 percent chromium, 4 to 46 percent nickel, 0.4 to 15 percent manganese, 0.015 to 0.75 percent sulfur and/or selenium, 0.5 to 10 percent copper, 0.25 to 2 percent aluminum, 0.001 to 0.1 percent tellurium, and the balance being iron, optional elements and incidental impurities. Austenitic compositions are provided having improved free-machining properties or an improved combination of free-machining and corrosion-resistance properties in selected media.

United States Patent [191 Goda, Jr. et a1. 7 i

1 a 1451 May21, 1974 1 FREE .MACHINING AUSTENITIC STAINLESS STEEL ALLOY[75] Inventors: Kermit J. Goda, Jr., Leesport;

. Grant M. Aulenbach, Mohnton,

both of Pa. [73] Assignee: Carpenter Technology Corporation,

. Reading, Pa.

[22] Filed: Jan. 24, 1972 21 Appl. N0.: 220,108

Related US. Application Data [63] Continuation-impart of Ser. No.855,416, Sept. 4,

1969. Pat. NO. 3,645,722.

UNITEDQSTATES PATENTS .R23,685 7/1953 Clarke ..75/1 2s1 3,152,934,10/1964 Lula ..75/124 1127.226 1/1970 MOSkOWitZ 75/128 P 2,661,27912/1953 Wilcox 75/128 P 3,362,813 l/l968 ZiOlkOWSki 75/124 I 1,961,7776/1934 Palmer 75/128 P 2,697,035 12/1954 Clarke 75/128 P 3,151,978 710/1964 .Perry 75/12'4 3,340,046 9/1967 Dulis 75/128 P 10/1968 Myers75/124 Primary 'Examirier-Hyland Bizot Attorney, Agent, or Firm-Edgar N.Jay

{57 ABSTRACT Austenitic stainless steel alloy containing about 10percent to 26 percent chromium, 4 to 46 percent nickel, 0.4 to 15percent manganese, 0.015 to 0.75

. percent sulfur and/or selenium, 0.5 to 10 percent cop- 12 Claims, NoDrawings FREE MACHINING AUSTENITIC STAINLESS STEEL ALLOY Thisapplication is a continuation-in-part of our copending application filedSept. 4, 1969, Ser. No.

r 855,416 and now US. Pat. No. 3,645,722, granted to modify standardgrades by means of free-machining additives for use where theaccompanying reduction in corrosion resistance couldbe tolerated. Forexample, A.l.S.l. type 303 and type 303(Se) are illustrative of gradeswhich contain respectively substantial amounts of sulfur andselenium,that is more than about 0.15 percent and usually 0.3 percent or more,for improved machinability. r

' .A number of other elements have also been used in combination with orin place of the elements-sulfur and selenium, but the results left muchto be desired, usually because the increase in the cost of the alloyand/or the detrimental effect on other properties such as corrosionresistance were not sufficiently offset by the gain in free-machiningproperties. Our present invention stems from our discovery that when arelatively small amount of tellurium is present together with theelements manganese, copper and aluminum, they work together with sulfuror. selenium or both of them to impart a unique degree of freemachinability to austenitic chromium-nickel stainless steel alloyswithout reducing the corrosion resistance to the extent expected fromprevious experience with the sulfur or selenium additions required toattain a like degree of machinability.

lt is therefore a principal object of the present inven. tion'to providean austenitic stainless steel characterized by an enhanced degree offree machinability compared to currently available corresponding gradesof austenitic free-machining stainless steel and having at least aboutas good corrosion resistance.

By providing an austenitic chromium-nickel stainlesssteel-alloycontaining the elements indicated in the fol-' lowing broad.range, much-of the foregoing objects are achieved while the preferedrange ensures attainment of the foregoing as well as additional objects,both ranges being given in approximate weight percent:

Broad Preferred Range Range Carbon up to 0.25 up to 0.2 Chromium 10-26lO-ZO Nickel 4-46 I 4-35 Manganese 0.4-J5 0.4-2 (Sulfur and/or0.0l5-0.75 0.02-0.35 Selenium) Copper 0.5-l0 0.75-4 Aluminum 0.25-2'0.5-l .25 Tellurium 0.00l-0.l 0.0 l-0.06

In addition, there can be included varying amounts of other elements, asfor example: up to about 3 percent silicon but preferably no more thanabout 1 percent, up to about 0.5 percent preferably no more than 0.035percent phosphorus, up to about 3.5 percent molybde-v num, or tungstencan replace all or part of the molybdenum in the ratio of about 2 m 1,up to about 0.01 percent boron, columbium up to about 10 times thepercent'carbon plus nitrogen but no more than about 2 percent, titaniumin an amount up to about equal to six times the percent carbon plusnitrogen but no more than about 1.2 percent, and up to about 0.35percent nitrogen. Except for incidental impurities, the balance of thealloy is iron.

The elements sulfur and/or selenium together with controlled amounts ofmanganese, copper, aluminum and tellurium work together in ourcomposition to provide an unexpected degree of free machinability andcorrosion resistance. While sulfur in amounts ranging I from about 0.015to 0.75 percent can be present, above about 0.5 percent increasingdifficulty ,may be encountered in both hot and cold working thecomposition. Therefore, when necessary to minimize working difficulties,we limit sulfur to no more'than about 0.4 percent and preferably to nomore than about 0.35 percent. Selenium on a one-for-one basis can besubstituted for all or part of the sulfur in our composition as isindicated in the foregoing tabulation where the ranges'stated are to beread as the broad and preferred amounts of the combined content of bothsulfur and selenium. The elements sulfur and selenium, individually ortogether, are not equivalent to'and cannot be substituted for theelement tellurium in'our composition.

For its effect in contributing to free machinability, manganesepreferably in an amount from about 0.4 to 2 percent is included in ourcomposition, while best results can be obtained with about 1.4 to 1.6percent. As is well known, manganese can be used in addition to or inplace of part of the nickel content in establishing the austeniticbalance in austenitic stainless steel alloys, and up to about 15 percentmanganese can be used for that. purpose inour alloy. Having in mind thatfreemachining stainless steel alloys are not intended for use where theywould be exposed to very corrosive media, the .best combination ofcorrosion resistance and free machinability is obtained with about 0.4percent to the latter are present). The passivation treatment serves toremove the manganese sulfides (or selenides) from the surface of thetreated part. Of course, if the passivated surface from which themanganese sulfides have been removed wears away in use, then the underlying material becomes exposed and is subject to attack.

Copper is present in solid solution in our composition and primarilyappears to enhance the corrosion resistance of our alloy. The effect ofcopper on free machinability seems to result from lowering the workhardening rate. For this purpose we preferablyuse about 0.75 to 4percent copper. Below about 0.5 percent, there is presence of aneffective amount of tellurium. Because .aluminum is a powerful ferriteformer, its use must be carefully controlled in compositions like thepresent which are to be entirely or primarily austenitic. Broadly, fromabout 0.25 to 2 percent aluminum can be present in our composition.Larger amounts of aluminum make it increasingly difficult to preservethe austenitic balance of our composition. We prefer to use aluminum inan amount ranging from about 0.50 to l.25 percent. v

In the absence of at least a small but effective amount of tellurium,the outstanding free machinability of our composition cannot beattained. We preferably use about 0.01 to 0.06 percent telluriumalthough as little as 0.005 percent or even 0.001 percent can have aneffect and as much as 0.1 percent tellurium can be used to someadvantage. In spite of the fact that tellurium in the presence of sulfurbut without the necessary amounts of manganese, aluminum and copper doesnot improve the free machinability of austenitic stainless steel andappears to detract from it, we have found that with the aluminum,manganese and copper present in our composition together with tellurium,there is an unexpected synergistic effect upon the free machinability ofour composition as measured by the drill test.

We do not have a complete explanation for or completely understand thephenomenon, but based on our instances with some improvement inmachinability. On the other hand, when the previously acceptable sulfurlevel is maintained, the addition of the elements manganese, aluminum,copper and tellurium, all within the 5 stated ranges, makes possiblesignificantly improved free machinability.

The examples having the analyses in approximate weight percent as shownin Table I serve to illustrate our invention.

Ingots were cast of each of the foregoing examples,

hot worked and shaped to form test pieces which were annealed beforetesting. Our composition is wholly or predominantly austenitic both atroom temperature and at the usual hot working temperature range for 5such alloys, that is from a furnace temperature of about 2,100 F.Preferably, ingots are first subjected to some light hot working tobreak up the as-cast structure. Then after surface preparation, e.g.,grinding to re-' move surface checks, hot working is completed in theusual way from a furnace temperature of about 2,100

F. In the case of more difficult to work ingots, somewhat better resultscan be attained by saddening at a higher temperature, up to about 2,300F.

Examples l-6 were annealed at about 1,950 F for one hour followed bycooling in air. The hardness of the examples in their annealed'conditionwas measured on the Rockwell B Scale, and the results are recorded atthe bottom of Table l.

The machinability of the specimens of each of the examples wasdetermined as the average depth of penetration in thousandths of an inchinto the specimens under carefully controlled conditions. While there isno 4 generally accepted standard for measuring machinability, the freemachining values were obtained by meaobservations, it appears that thetellurium forms telluring the epth f Penetration into the specimens byum-rich compounds which we thus far have identified as tellurides thatare attached to the sulfides. The more tellurium present, the more thesulfides are surrounded by the tellurium-rich compounds.

By the addition of the elements manganese, aluminum, copper andtellurium in accordance with our invention, the sulfur content of agiven alloy ,can be reduced so as to obtain better corrosion resistancewith little or no loss in free machinability or even in some aquarter-inch drill in a time interval of 15 seconds with the drillrotatingat or very close to 670 rpm. under constant torque. Before thestart of each drilling operation, the drill mounted in a conventionaldrill press was 40 brought against the surface of the specimen where it7, TABLE 1 EXAMPLE NO. 1 3 4 5 6 c .120 091* 120* .131 .095 .095 Mn 1.721.63 1.68 1.68 1.67 1.68 Si .45 .51* .39 .44 44* P .005 .012 005* .012.012 012* s .31 .27 .31 .27 .27 .24 c: 17.26 16.63 I716 16.82 17.1417.14 Ni 10.00 10.06 1000* 10.08 9.81 9.81 Mo .01 .01 .01* .01 01* .01-Al .84 .50 .67 .41 .92 .92 Cu .85 1.47 1.68 .79 .74 1.49 Te .001 .004.016 .04 .04 .04 N .037 .046 .037 .032 .042 .042 Rockwell B 79.5 78 I 8186 84.5

Drill Test" 547 For economy. the heats were split to permit variation inthe elements Mn. S. Al. Cu and Te. The asterisk indicates analyses ofelements obtained by measurement from other portions of the same heat toavoid unnecessary analysis.

"In thousandths of an inch.

. rium-is not added in accordance with our invention. On

For comparison, the following alloys were prepared, (H)! formed intospecimens and tested as was described in connection with Examples l-6.Nitrogen do. 0.35

TABLE 11 T A B c .D

c .101 .107 .108" .115- Mn .07 1.47 1.76 1.69 st .24 .31 43* .38

P .007 .007 006* .006 s .37 .42 .32 .31 c: 16.95 17.02 17.19* 17.09 Ni9.76 9.79 979* 9.79 MO .02 .02 02* .01 Al .0l .0l .36 .92 c .05 .06 .761.54 Te .0002 .0002 .0002 .o01 N, .043 .045 .036 .038 Rockwell B 88.5 8885 81.5 D1111 Test 338 429 423 443 The asterisk indicates analyses ofelements obtained the same heat to avoid-unnecessary, analysis. "Inthousandlhs of an inch.

Alloy A shows the effect of sulfur in such a'chromium-nickel austeniticstainless steel with low manganese and without an addition of aluminum,copper and tellurium. With a hardness of R 88.5 in the annealedcondition, the average drill test penetration of Alloy A was found to be.338 inch. Alloy B'primarily demonstrates the effect of adding manganesein increasing the average drill test penetration to .429 inch, but theincrease in sulfur from .37 to 42 percent also contributed itsrelatively small effect. Alloys C and D demonstrate that the addition ofthe elements aluminum and copper does not provide a synergistic effectwith manganese and could result in a reduction in free machinability iftelluthe other hand, Example I, which has virtually the same analysis asAlloy D except for the small addition of tellurium, gave an' averagedrill test penetration of .547 inch. See also Example 2 which, with only.27 percent sulfur and .004 percent tellurium, gave an average drilltest penetration of .455 inch. The as-tested hardness of the specimensof Examples 1 and 2, and Alloy D ranged from R ,79.5 to R ,8l.5 andthus'were not considered to differ significantly.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such tenns and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

1. An austenitic stainless steel alloyhaving good free machinability inits annealed condition consisting essentiallyin weight percent of about:

Chromium 10-26 Nickel 4-46 Sulfur plus Selenium 0.0l50.75

Copper 0.5-l0 Aluminum 0.25-2

Tellurium 0001-0. 1

Manganese 0.4-15

Carbon 4 4 up to about 0.25

Silicon 1 do. 3

Phosphorus do. 0.5 Molybdenum do. 3.5 5

by measurement from other portions of in which an equivalent amount ofup to about 7 percent tungsten in the ratio of about 2/] can replace allor part of the molybdenum, and the balance essentially ir'on.

2. The austenitic stainlesssteel alloy as set forth in claim 1containing about 0.02-0.5 percent sulfur plus selenium, about 0.4-2percent manganese, about 0.75-4 percent copper, about 05-125 percentaluminum, and about 0.005-006 percent tellurium.

3. The austenitic stainless steel alloy as set forth in claim 2containing about 1.4-1.6 percent manganese.

4. The austenitic stainless steel alloy as set forth in claim 1containing about 0.4-2 percent manganese.

5. The austenitic stainless steel alloy as set forth in claim 4containing about 0.5-4 percent copper.

6. The austenitic stainless steel alloy as set forth in claim 5containing about 10-20 percent chromium, and about 4-35 percent nickel.

7. The austenitic stainless steel alloy as set forth in claim 6containing no more than about 1 percent silicon, and no more than about0.035 percent phosphorus.

8. The austenitic stainless steel alloy as set forth in claim 2containing about Weight Percent Chromium l0-20 Nickel 4-35 Carbon up toabout 0.2 Silicon do. I Phosphorus do. 0.035 Columbium do. I l0 C+N)Titanium do. 6 C+N) 9. "11..'zntesitizstani s 844511575.4 1.51. 1;.

claim 11 containing about 0.4-0.7 percent manganese. l

2. The austenitic stainless steel alloy as set forth in claim 1containing about 0.02-0.5 percent sulfur plus selenium, about 0.4-2percent manganese, about 0.75-4 percent copper, about 0.5-1.25 percentaluminum, and about 0.005-0.06 percent tellurium.
 3. The austeniticstainless steel alloy as set forth in claim 2 containing about 1.4-1.6percent manganese.
 4. The austenitic stainless steel alloy as set forthin claim 1 containing about 0.4-2 percent manganese.
 5. The austeniticstainless steel alloy as set forth in claim 4 containing about 0.5-4percent copper.
 6. The austenitic stainless steel alloy as set forth inclaim 5 containing about 10-20 percent chromium, and about 4-35 percentnickel.
 7. The austenitic stainless steel alloy as set forth in claim 6containing no more than about 1 percent silicon, and no more than about0.035 percent phosphorus.
 8. The austenitic stainless steel alloy as setforth in claim 2 containing about
 9. The austenitic stainless steelalloy as set forth in claim 8 containing about 1.4-1.6 percentmanganese.
 10. The austenitic stainless steel alloy as set forth inclaim 8 containing about 0.02-0.35 percent sulfur plus selenium.
 11. Theaustenitic stainless steel alloy as set forth in claim 10 containingabout 0.01-0.06 percent tellurium.
 12. The austenitic stainless steelalloy as set forth in claim 11 containing about 0.4-0.7 percentmanganese.